The EarthBox is based on ideas developed for commercial farmers to ensure good yield even when conditions are not ideal (like on a rooftop...). The design makes planting simple - you are forced to add only the right amount of water and soil. Instructions are specific on how many plants to begin per box, depending on what you want to grow. A water reservoir with an overflow hole means you won't over water, and a mulch cover protects the soil/fertilizer from wind and intense sun (thinking rooftop again).

We went to the Chicago Flower and Garden show a few weeks ago and spent a long time talking with the Earthbox people about why these work. Many sample boxes were set up; the variety and health of the plants growing in them was remarkable.
Another good thing: they are part of a very cool program, The Growing Connection which is setting these up around the world for young people in places where it was very hard to grow veggies before - from inner city Chicago to Ghana.

We'll be keeping an eye on Raised on the Roof over the next couple of months for a test of a real-time Chicago growing season - just how many brussels sprouts can he grow on that roof anyway? Watch for the updates...